1. Field of Invention
This invention relates generally to burners, and more particularly to burners designed to generate heat for a variety of uses by combusting particulate fuels such as sawdust, pelletized refuse, and the like or burners designed to dispose of excess materials simply by clean incineration.
2. Description of Prior Art.
There are many occasions in industry for the efficient combustion of particulate material. For example, in the lumber industry there is a need for a means to burn sawdust for steam generation and/or disposal. Where the fuels are readily available, home burners for space and water heating are commonly used. With increasingly strict air pollution laws, especially in those states with a heavy lumber industry, there is a need for a burner that combusts the sawdust efficiently with little or no smoke or fly ash. Such a burner must also be able to function safely in less than ideal conditions in which wet fuel or sawdust must be burned.
A commonly used burner for this purpose comprises a combustion chamber having a top-mounted hopper which feeds the fuel directly onto a series of grates within the combustion chamber stacked in stairstep fashion. As the fuel flows down the grates to the bottom of the combustion chamber, it mixes with combustion air drawn through the grates and burns. This air can be provided to the combustion chamber by the natural draft created when the fuel burns and the hot combustion gases exiting the combustion chamber draw air into the combustion chamber through perforations in or openings between the grates. Examples of this type of incinerator are found in U.S. Pat. Nos. 1,641,545; 1,840,668; and 2,183,648. Another method of supplying combustion air is by forcing it through the fuel by the use of air blowers located within the grates or mounted in the walls of the combustion chamber. This also agitates the fuel and urges it down the grates. An example of this method is found in U.S. Pat. No. 3,812,794.
An inherent problem in burning particulate fuels in this manner is the tendency of the fuel to burn out locally generating holes and pillars--clumps of charred, sintered fuel--along the width of the grates. The pillars hold back the fuel preventing it from filling the holes. When the pillars weaken and collapse, a rush of fuel cascades down the grates burying the partially burned fuel. This generates smoke and gases which can accumulate in the combustion chamber and explode, resulting in damage to the burner and injury to bystanders. Attempts in prior art devices to prevent pillaring by spreading the fuel across the grate have resulted in the accumulation of unburned fuel in corners located where the edge of a grate meets a wall of the combustion chamber and is caused by the relative absence of air in these corners.
Therefore, it is an object of the invention to provide a particulate fuel burner utilizing grates stacked in stairstep fashion in which the fuel flows continuously down the grates as it burns. This will greatly reduce pillaring of the fuel on the grates and will virtually eliminate intermittent cascading of the fuel and with it the attendant smothering of the fire, accumulation of explosive gases within the combustion chamber, and generation of excess smoke.
It is also an object of the invention to provide a burner in which the fuel is distributed across the grates so that it flows downward in a manner that promotes even mixing with the air, a reduction in pillaring, and has a fire-sustaining motion down the grates.
Another object of the invention is to provide a burner in which air for combustion can bypass the flow of fuel down the grates to be supplied directly to fuel located on a grate adjacent a wall of the combustion chamber so that burning is maintained on each grate at all times and so that air is available beyond the grates to assure complete combustion of all flammable gases and fumes.
An additional object of the invention is to provide a burner in which the rate of flow of fuel down the grates can be varied by adjusting the position of the grates relative to one another to vary the slope down which the fuel flows, thus compensating for slower burning fuel such as wet sawdust.
It is also an object of the invention to provide a burner which contains an easily adjustable means for bringing air to unburned particles that collect beyond the base of the lowermost grate.
It is a further object of this invention to provide a burner in which air for combustion may be supplied either by natural draft or by mechanically forced air, although natural draft is preferred.
It is also an object of the invention to provide a burner which operates efficiently without the use of outside power or sophisticated electronic controls and whose grates can be constructed of relatively inexpensive plate or sheet metal parts.
Another object of the invention is to provide a burner that easily can be retrofitted into presently existing boiler, heater, and incinerator systems to bring them into compliance with Federal emission regulations.